More than 5 million Americans have now signed up for marketplace health coverage on state and federal insurance exchanges, the Obama administration announced Monday.

The Congressional Budget Office expected six million people to sign up for coverage by the time open enrollment ends on March 31. But research by consulting firm, Avalere Health, suggests the final enrollment tally may only reach 5.4 million people.

With just two weeks left to purchase coverage that complies with the Affordable Care Act, consumers are swamping the federal website and its call centers. The last few days were the busiest since December, as the federal call center took more than 198,000 calls on Thursday alone. That was the busiest day since December 23. Another 130,000 calls were fielded over the weekend., the federal insurance marketplace, had more than 4 million visits last week and another 1 million this past weekend, according to a blog post on Monday by Medicare Administrator Marilyn Tavenner.